It is known in the plastic molding art to use pressurized fluid in conjunction with the plastic molding of articles, as disclosed in the Friederich U.S. Pat. No. 4,101,617.
Gas-assisted injection molding is a thermoplastic molding process which provides stress-free large parts with a class A surface and virtually no sink marks. Gas-assisted injection molding is a low-pressure molding process compared to conventional injection molding. In this process, inert gas is injected into the plastic after it enters the mold. The gas does not mix with the plastic but remains in the middle of the thicker sections of the molding. By controlling the gas pressure, the quantity of plastic injected into the mold (short shot) and the rate of gas flow, a predetermined network of hollow interconnecting channels is formed within the molded part. The gas pressure remains constant in the network of hollow channels throughout the molding. This compensates for the tendency of the plastic to shrink at the thicker areas of the molding, preventing warpage and reducing stress. The gas pressure is relieved just prior to opening the mold. Because of the relatively low injection pressure, large parts can be molded with substantial reductions in clamp tonnage.
The gas system equipment provides the precise control of pressure, timing and volume of gas which is injected into the part, all of which are important to the control of the gas-assisted injection process.
In U.S. Pat. No. 4,948,547 entitled "Improved Method for the Use of Gas Assistance in the Molding of Thermoplastic Articles," assigned to the Assignee of the present invention, a method of gas-assisted injection molding is disclosed in which a charge of pressurized gas is injected into the mold but not into the article-defining cavity. The gas charge is of a predetermined quantity and pressure, sufficient to assist in filling out the article defining cavity with resin and promoting surface quality.
FIG. 2 is a general schematic view of a prior art apparatus suited for practicing plastic injection molding, generally of the type of which the present invention is directed.
The controlled entry of pressurized fluid, typically nitrogen gas, is accomplished by the use of a modified mold sprue 10. The sprue 10 includes a disc-shaped insert 12 disposed within a sprue body 14.
The mold sprue 10 cooperates with a conventional plastic injection molding machine 20, the nozzle 18 of the molding machine 20 mates with a concave surface 22 on the face of the insert to provide a continuous path 16 for the flow of plastic from the machine 20 through the sprue 10 and into a mold cavity (not shown).
The flow of molten plastic through the insert 10 may be diverted by a conventional torpedo 4 of the type well known in the art.
The introduction of pressurized fluid to the flow path is through passage segments 26 and 28 formed (by drilling or the like) in the insert. The passage 26 opens into the flow path through an orifice 30 of sufficiently small dimension, for example, 0.005 to 0.040 inches, depending on the viscosity of the plastic to actively prevent entry of the relatively high viscous molten plastic during injection.
The plastic injection molding machine 20 includes a barrel 32 with a central cylindrical opening 34. A screw 36 serves to plasticize and advance resin toward the nozzle area. Upon complete plasticization of the resin, the screw 36 is hydraulically advanced toward the head of the barrel 32 to inject molten plastic through the nozzle 18. The plastic passes through the sprue insert 12 at a nominal plastic injection pressure through the stroke of the screw 36. This pressure falls upon substantial completion of the stroke and discharge of the plastic from the barrel 32 of the molding machine 20.
The insert 12 is shown mounted concentrically in a recess in the sprue body 14. Molten plastic passes from the nozzle 18 and around the torpedo through a pair of kidney-shaped apertures (not shown) which serves as first and second branches in the flow path. The pressurized fluid is communicated to the plastic flow path through passage segment 26 and orifice 30 which is mediate the plastic flow branches and colinear therewith.
The temperature of the insert 12 can be controlled, depending on the processing specification of the plastic being used by employing electrical heater bands or other types of auxiliary heat sources, as is well known in the art.
The apparatus of FIG. 2 also includes a mechanism 37 for charging and communicating the pressurized fluid or gas to the sprue insert 12. For example, a hydraulic fluid supply 38 directs a working fluid, such as oil under pressure to a chamber 40 of an accumulator 42 effectively separated into two chambers, having mutually and inversely variable volumes by a compression piston 44. A fluid supply 46 is provided for directing a charge of gas through a first check valve 48 into the second chamber 50 of the accumulator 42 for pressurization. A control valve 54 controls communication of the gas from the chamber 50 to the sprue insert 12. A check valve 52 is connected in series with the control valve 54.
The mechanism for charging a pressurized fluid or gas for use in the prior art molding process is described in greater detail in U.S. Pat. No. 4,855,094. Also, a detailed description of the operation of the mechanism 37 is described in this patent which is assigned to the Assignee of the present application.
One limitation of the prior art mechanism 37 is that the hydraulic unit can only recharge after the plastic injection molding cycle is substantially completed (typically 75%). Also, such a hydraulic unit must be recharged after each cycle. Consequently, a relatively constant pressure is not always available with such a hydraulic unit which uses a multiplier system.
Another drawback of such a hydraulic unit is that it is not flexible to adapt to more than one concurrently operating molding process. Consequently, a separate hydraulic unit must be provided for each injection molding machine and mold combination.